i) Field of the invention
The present invention relates to a data conversion method for converting digital data into suitable signals for a recording or transmission system in recording or transmitting the digital data, a pilot signal formation method using the data conversion method for obtaining a tracking error signal in a magnetic recording and reproducing apparatus such as a digital VTR (video tape recorder) system or the like, and a rotary magnetic head device for use in the magnetic recording and reproducing apparatus.
ii) Description of the related arts
A conventional data conversion system, for example, an 8/10 modulation system has been developed, as disclosed in "The Dat Conference Standard", June, 1987.
In the conventional 8/10 modulation data conversion system, digital data is delimited by data words of 8 bits, and the data words are converted into code words of 10 bits, as shown in FIGS. 1 to 3. In FIG. 1, a data word a of 8 bits and a first table selection signal Q' of 1 bit are input to an encoder 1, and the encoder 1 outputs a code word b of 10 bits and a second table selection signal Q of 1 bit for a next code word. A flip flop 2 receives the second table selection signal Q for the code word b and delays the table selection signal Q for one data word a
In the encoder 1, for instance, a data conversion table shown in FIG. 2 for converting the data words into code words is stored in a ROM (read only memory) or the like. In the data conversion table, 256 data words of hexadecimal codes of "00" to "FF" correspond to code words of CDS (code word digital sum)=0 in one-to-one relation and also to the code words of CDS.noteq.0 with reference to a pair of CDS values of +2 and -2, and tables Q'=-1 and Q=+1 are composed of data words of CDS=+2 and CDS=-2, respectively. The signal Q selects the CDS (table) so as to suppress divergence of charges in a code word string.
In FIG. 3, signals a, b and Q correspond to those at the points a, b and Q in FIG. 1, and a signal c is obtained after a NRZI (non-return-to-zero inverse) modulation where inversion is carried out by data "1". A signal d represents DSV (digital sum variation) at the end of each code word after the NRZI modulation.
The operation of the 8/10 modulation system described above will now be described in detail.
First, when an 8 bit data word a of FF and a first table selection signal Q'=-1 are input to the encoder 1, the encoder 1 outputs a 10 bit code word b of 1111101010 having a CDS=+2 corresponding to FF of the signal Q'=-1 and a second table selection signal Q=-1. Then, the 10 bit signal is converted from a parallel signal into a serial signal, and the NRZI modulation of the serial signal is carried out. Hence, the DSV value of the end of the code word becomes +2.
Then, when a data word a of 00 is input to the encoder 1, a 10 bit code word b of 0101010101 having a CDS=0 corresponding to 00 of the signal Q'=-1 resulted from delaying the just preceding output signal Q=-1 by one symbol in the flip flop 2, and a signal Q=1. As a result, the DSV value of the end of the code word b after the NRZI modulation is +2.
Next, when a data word a of 11 is input to the encoder 1, a 10 bit code word b having a CDS=-2 corresponding to 11 of the signal Q'=1 and a signal Q =-1. Accordingly, the DSV value of the end of the code word b after the NRZI modulation is 0. That is, in general, when an 8 bit data word a is input to the encoder 1, the encoder 1 outputs a code word b selected from either the table where Q'=-1 or Q'=1 corresponding to the data word a according to a first table selection signal Q output right before, and as a result, a DSV value at the end of each code word b after the NRZI modulation is restricted to 0 or .+-.2 n. This means that the divergence of the DSV is suppressed, and as a result, a DC free data conversion excluding any direct current component can be realized.
In the conventional data conversion system, as described above, since the CDS values of the code words obtained in the conversion are selected from only 0 and .+-.2, the suppression control of the DSV values for the code words can not be positively or actively carried out, and the spectrum of the code word includes relatively low frequency components. Further, when a DSV control circuit is provided to use a DSV value as one of data, the DSV value can not be controlled every code word.
In FIGS. 4 and 5, there is shown a pilot signal formation circuit and a tracking error detecting circuit for use in producing a tracking error in a conventional magnetic recording and reproducing apparatus, as disclosed in Japanese patent laid-open No. Sho 59-68862. As shown in FIG. 4, a reference oscillator 101 for generating a reference signal, a presetable counter 102, a flip flop 103, a filter 104 and a mixer 105 for adding a reference sine wave signal 106 output from the filter 104 and a data signal 107 representing an audio or visual signals are connected in series. A magnetic head 109 for carrying out the recording or reproducing of a signal onto or from a magnetic medium 122, such as a magnetic tape, is coupled with the mixer 105 through a turnover switch 108 for selecting recording or reproducing, and a frequency dividing ratio controller circuit 10 receives a track switch signal 111 and a record and reproduction switch signal 112 and controls the frequency dividing ratio of the presetable counter 102.
A low pass filter 113 for inputting a reproduction signal 114 fed from the magnetic head 109 via the turnover switch 108, a mixer 115 for adding the reproduction pilot signal output from the low pass filter 113 and the reference signal 106 fed from the filter 104, an amplifier 116 and a divider circuit 117 are connected in series. The divider circuit 117 outputs a signal to a pair of envelope detector circuits 119a and 119b through respective band pass filters 118a and 118b, and a differential amplifier 120 receives the output signals of the two envelope detector circuits 119a and 119b and compares them to output a tracking control signal 121. FIG. 5 illustrates the magnetic medium 122 such as the magnetic tape and the magnetic head 109 which is movable along recording tracks 123 on the magnetic tape in the conventional magnetic recording and reproducing apparatus.
The operation of the conventional magnetic recording and reproducing system shown in FIGS. 4 and 5 will now be described in detail.
First, in the recording of a signal onto a magnetic tape, the frequency dividing ratio of the presetable counter 102 is switched by the frequency dividing ratio controller circuit 110 according to the track switch signal 111, and the output signal of the presetable counter 102 is further frequency-divided by the flip flop 103. The filter 104 receives the output signal of the flip flop 103 and outputs the reference sine wave signal (pilot signal) 106 to the mixer 105, and the mixer 105 adds the reference signal 105 and the data signal 107 to output a recording signal to the magnetic head 109 via the turnover switch 108. The magnetic head 109 records the recording signal onto the magnetic tape 122.
In this case, since the track switch signal 111 is changed every time the recording track is changed, for example, four kinds of pilot signals f1, f2, f3 and f4 can be recorded onto the magnetic tape, as shown in FIG. 5. In this instance, it is necessary to determine the frequencies of the pilot signals from, for instance, several tens of kHz to several hundreds of kHz, so that the data signal 107 may not be damaged when the pilot signal is extracted and the data signal 107 is reproduced.
By determining the frequencies of the four pilot signals f1 to f4 to the following formulas in consideration of a 4 frequency pilot system of an 8 mm VTR (video tape recorder) for public use, EQU f1+fA=f2, f2+fB=f3 (1) EQU f4+fA=f3, f1+fB=f4 (2)
when the recording signal is reproduced from the magnetic tape by the magnetic head 109 shown in FIG. 4 in a reproducing mode, the pilot signal mixed with the data signal 107 recorded onto the magnetic tape is also reproduced. This pilot signal can be extracted by the low pass filter 113, and at this time, not only the pilot signal for the track now being scanned by the magnetic head 109 but also the pilot signals of both adjacent tracks thereto are picked up as crosstalk.
Since the frequency of the pilot signals of the adjacent tracks is low enough compared with the video signal or the like, for example, even in an azimuth recording, the azimuth effect will be negligible, and thus the pilot signals of the adjacent tracks can be reproduced as a large crosstalk amount. When the pilot frequency of the reference signal 106 written in the scanning track is added to the pilot signals reproduced as above in the mixer 115, a beat is caused between the reference signal 106 and the pilot signals due to the crosstalk of the adjacent tracks, and beat frequencies of the beat signals fA and fB in formula (1) above-described can be obtained.
As shown in FIG. 5, for instance, on reproducing the track 123 in which the pilot signal having the frequency f2 is written, the pilot signals having the frequencies f1 and f3 can also be obtained as crosstalk, and, when the pilot signals are added to the reference signal 106 in the mixer 115, the beat signals fA and fB can be obtained from above formulas (1) and (2) such as f2-f1=fA and f2-f3=-fB.
Next, the output signal of the mixer 115 is fed through the amplifier 116 and the divider circuits 117 and is extracted in the band pass filters 118a and 118b. Then, the filtered signals are detected in the envelope detector circuits 119a and 119b. At this time, while the magnetic head 109 scans on-track along the track of the signal f2, when the magnetic head 109 is shifted a slight amount toward the f1 side, the beat signal fA increases, or a slight amount toward the f3 side, the beat signal fB increases, and hence the output signal of the differential amplifier 120 can be output as the tracking control signal 121.
In the conventional magnetic recording and reproducing apparatus as described above, high density recording or reproducing is carried out, and a tracking system with extremely narrow tracks is provided. Hence, in this case, a device capable of detecting a track shift with high accuracy is required, and in general, as described above, by recording the low frequency pilot signals, the track shift can be detected. However, in the case of digital magnetic recording, there is a power spectrum extending over a wide frequency range from near direct current to a maximum recording frequency in usual recording and reproducing, and thus a gap in the so-called frequency allocation can not be formed outside the range of a carrier and its periphery as in conventional analog FM recording. In particular, in a digital recording, it is difficult to insert a low frequency pilot signal for tracking into a gap in the frequency allocation like present analog 8 mm VTR.
When the power level of the pilot signal recorded in the frequency range of the pilot signal for tracking is large enough compared with the power level of the recording signal obtained by modulating the digital data even in the digital recording, the pilot signal for tracking can be extracted by a band pass filter or the like and reproduced in the same manner as a conventional example.
However, in the case where the power level of the pilot signal is enlarged too much with reference to the recording or reproducing signal of visual or audio data as described above, when the signal is demodulated during reproducing, the wave form deformation is enlarged and the error rate of the digital data increases. Particularly, when the digital data after the modulation and the pilot signal for tracking are added in an analog way before being input to the recording amplifier, since there is no relationship between the digital data and the pilot signal, the two signals mutually act as only disturbance signals.
That is, in a digital data recording and reproducing apparatus such as a digital audio recorder or a digital video recorder, the frequency spectrum of a recording or reproducing signal includes many low frequency components due to a feature of digital recording, and, when a low frequency pilot signal for tracking is added to the recording or reproducing signal to record the added signal, since there is no relationship between the recording or reproducing signal and the pilot signal, a wave form deformation is caused when demodulating the modulated digital signal, and the data error rate increases.
In order to reduce the wave form deformation caused in the demodulating, the power level of the pilot signal is lowered, and a necessary S/N ratio for a servo (tracking) detection signal can not be obtained. Accordingly, the servo can not be given, and the recording density in the tracking direction in the magnetic tape can not be gained.
In FIG. 6, there is shown a conventional rotary magnetic head device, as disclosed in Japanese patent laid-open No. Sho 58-47383. FIG. 7 shows a track pattern recorded on a recording medium such as a magnetic tape by the rotary magnetic head device shown in FIG. 6, and in this instance, the recording is carried out without any guard band, adjacent two tracks having different azimuth angles. As shown in FIG. 6, two double azimuth heads 202 each composed of a pair of heads H.sub.L1 and H.sub.H1 or H.sub.L2 and H.sub.H2 having different azimuth angles are arranged on the periphery of a rotary drum 201. The pairs of heads H.sub.L1, H.sub.H1, H.sub.L2 and H.sub.H2 are aligned in opposite positions through 180.degree. with reference to the central axis of the rotary drum 201, and the pair of heads H.sub.L1 and H.sub.H1, or H.sub.H1 and H.sub.H2 are arranged at a distance away from each other corresponding to 6H (H means a horizontal scanning period) time. A recording medium 203 such as a magnetic tape is wound around approximately half the rotary drum 201.
The operation of the conventional rotary magnetic head device shown in FIGS. 6 and 7 will now be described in detail.
First, in a recording mode, a composite color signal composed of a luminance signal and a color signal multiplied by each other is divided into two system signals such as a low range signal SL including a signal representing a brightness component and a high range signal SH including a color signal component (carrier color signal) and a high range luminance signal component, and the low and high range signals SL and SH are frequency-modulated. Then, the modulated low and high range signals are input to the double azimuth head 202 composed of two heads H.sub.L1 and H.sub.H1 or H.sub.L2 and H.sub.H2 arranged on the rotary drum 201 and are recorded in two channels on the recording medium 203. In this instance, the divided two range signals SL and SH are passed through two different systems, and hence their delay times can be different on reproducing. Hence, a timing-axis adjustment may be required, and as reference signals for the timing-axis adjustment, a burst signal and a horizontal synchronizing pulse (PH) are used for the low range signal SL and the high range signal SH, respectively. The PH signal is a low frequency signal with negligible azimuth effect. As a result, as shown in FIG. 7, by determining the head distance between the two heads H.sub.L1 and H.sub.H1 or H.sub.L2 and H.sub.H2 to 6H, the H-alignment is achieved in the tracking pattern recorded on the recording medium 203. Hence, even when the crosstalk is increased in the P.sub.H portions by mistracking, such sections correspond to the horizontal blanking periods, and no image quality deterioration by the crosstalk will be caused.
However, in the conventional rotary head device as described above, when the head device is applied to the digital recording, there is no signal corresponding to the horizontal synchronizing signal, and thus an appropriate tracking error signal can not be obtained. Further a determination of the head interval in the double azimuth head is newly required.